DESCRIPTION: (Applicant's Abstract) The objective of this application is to determine whether activation of protein kinase C (PKC) is a viable strategy for anti-leukemic therapy. Acute myeloid leukemia (AML) and myelodysplasia (MDS) are manifested by bone marrow failure resulting from a partial or complete block in maturation of hematopoietic elements. The protein kinase C (PKC) family of calcium and phospholipid-dependent serine-threonine kinases plays a critical role in transducing extracellular signals involved in monocytic differentiation. The phorbol ester 12-0-tetradecanoyl-phorbol-13- acetate (TPA), a potent inducer of maturation along the monocytic lineage, binds to and activates PKC. The applicant demonstrated that all-trans retinoic acid (ATRA) treatment of the TPA-resistant HL-60 cell subline (HL- 525) markedly increases the relatively low constitutive levels of PKCb mRNA and protein found in this variant. The ATRA- mediated increase in PKC activity and PKCb gene product is associated with the acquisition of TPA responsiveness characterized by adherence, non-specific esterase staining, and c-fms gene expression. He has demonstrated that bryostatin 1 (a non-tumor promoting compound which activates PKC at nanomolar concentrations) treatment of ATRA- primed HL-60 and U-937 cells also leads to a marked augmentation of features of monocytic differentiation in these cells. Therefore, the combination of ATRA and bryostatin 1 may be useful in diseases characterized by a block in myeloid differentiation. This application will build upon the applicant's preliminary observations by defining the PKC isoform(s) which are critically increased by retinoic acid and subsequently activated by TPA or bryostatin 1 in several leukemia cell lines and human leukemic cells (Specific Aim 1). Downstream elements directly interacting with the relevant PKC isoform will be determined by means of glutathione-S-transferase fusion constructs (Specific Aim 2). To insure that his findings with leukemic cell lines are generalizable, he will test the retinoic acid/bryostatin 1 combination against human leukemias propagated in immunodeficient mice (Specific Aim 3). Finally, he will conduct a phase II trial of all trans retinoic acid in combination with bryostatin 1 in patients with MDS and refractory AML, correlating therapeutic response with studies to assess biologic effects of therapy including the ability of sera from treated patients to induce differentiation in vitro and in vivo (Specific Aim 4).